GEOLOGICAL MAPPING. 15 



could not meet. But the proposition is true to a 

 certain point only when the dip, in this direc- 

 tion, exceeds that of the slope, the boundary lines, 

 in this case also, begin to draw in towards a 

 straight line, which they must eventually attain 

 to if the dip increased until the strata were vertical. 

 Strata sometimes occur in a horizontal or nearly hori- 

 zontal position. It is evident from Prop. 1, that if we 

 can once fix a point through which passes the boundary 

 of such a stratum, a contour line drawn from this point 

 will accurately represent the boundary so far as its 

 horizontality is continued. 



Much more frequently we find strata dipping into the 

 higher ground from beneath which they have risen to 

 the surface. Indeed, this may be considered the normal 

 position of stratified rocks now forming dry land, as 

 their dip has itself given the initial form to the hills 

 above them. In this case any points at the same level 

 on the line of strike must be on the boundary assuming 

 its passing through one of them to have been ascertained. 

 A line following the curves of the contours, but flat- 

 tened in proportion to the dip (Prop. 2), represents 

 accurately the line required. 



It is an exceptional occurrence for strata, at their out- 

 crop as opposed to ' dip-slope,' to dip with the slope of 

 the ground. When this does occur, the line of junction 

 must be ascertained in several places, and the points 

 united by exaggeration of the contour (Prop. 3). 



Contours run in a V-like shape up the valleys, in 

 straight lines on flanks and ridges, and sweep round the 

 outline of the hills their variations are as numerous as 

 the hills themselves, but this kind of form prevails in 



